The present invention relates to a charge pump type DC/DC converter and a power supply apparatus for liquid crystal devices using the same.
As a conventional charge pump DC/DC converter (hereafter referred to as a first conventional apparatus), a double step-up complementary driving type shown in FIG. 6 is known.
The first conventional apparatus is equipped with, as shown in FIG. 6, a double step-up first charge pump circuit 1, a first driving circuit 2 that drives the first charge pump circuit 1, a double step-up second charge pump circuit 3, a second driving circuit 4 that drives the second charge pump circuit 3, an oscillation circuit 5 that oscillates a signal to be provided to the first driving circuit 2 and the second driving circuit 4, an input terminal 6 and an output terminal 7.
The first charge pump circuit 1 is formed from MOS transistors Q1 to Q4 and a capacitor C1. Also, the second charge pump circuit 12A is formed from switching MOS transistors Q5 to Q8 and a capacitor C2,
Next, operations of the first conventional apparatus having the structure described above are described with reference to FIG. 6 and FIGS. 7A and 7B.
In the first conventional apparatus, the first and second charge pump circuits 1 and 3 are placed in a state shown in FIG. 7A in a first period, and in a state shown in FIG. 7B in a second period. The operations in the first period and the second period are alternately repeated.
In other words, in the first period, in the first charge pump circuit 1, only the MOS transistors Q2 and Q4 are turned on by the first driving circuit 2, and the capacitor C1 is charged with an input DC voltage Vin (see FIG. 7A).
Also, in the same first period, in the second charge pump circuit 3, only the MOS transistors Q5 and Q7 are turned on by the second driving circuit 4, and a voltage in which a charged voltage of the capacitor C2 in the second period in a previous round is added to an input DC voltage Vin becomes to be an output DC voltage Vout (see FIG. 7A).
In contrast, in the second period, in the first charge pump circuit 1, only the MOS transistors Q1 and Q3 are turned on by the first driving circuit 2, and a voltage in which a charged voltage of the capacitor C1 in the first period in a previous round is added to an input DC voltage Vin becomes to be an output DC voltage Vout (see FIG. 7B).
Also, in the same second period, in the second charge pump circuit 3, only the MOS transistors Q6 and Q8 are turned on by the second driving circuit 4, and the capacitor C2 is charged with an input DC voltage Vin (see FIG. 7B).
Meanwhile, as a second example of a conventional charge pump DC/DC converter (hereafter referred to as a second conventional apparatus), a type shown in FIG. 8 is known.
The second conventional apparatus is equipped with, as shown in FIG. 8, a charge pump circuit 11, a driving circuit 12 that drives the charge pump circuit 11, an oscillator 13 that oscillates a specified signal to be supplied to the driving circuit 12, an input terminal 14 and an output terminal 15.
The charge pump circuit 11 is formed from MOS transistors Q11 to Q14, and a capacitor C11.
Next, operations of the second conventional apparatus having the structure described above are described with reference to FIG. 8 and FIGS. 9A and 9B.
In the second conventional apparatus, the charge pump circuit 11 is placed in a state shown in FIG. 9A in a first period, and in a state shown in FIG. 9B in a second period. The operations in the first period and the second period are alternately repeated.
More specifically, in the first period, in the first charge pump circuit 11, only the MOS transistors Q12 and Q14 are turned on by the driving circuit 12, and a capacitor C11 is charged with an input DC voltage Vin (see FIG. 9A).
On the other hand, in the second period, in the charge pump circuit 11, only the MOS transistors Q11 and Q13 are turned on by the driving circuit 12, and a voltage in which a charged voltage of the capacitor C1 in the first period is added to an input DC voltage Vin becomes to be an output DC voltage Vout (see FIG. 9B).
It is noted that the first conventional apparatus is a complementary driving type, which is effective in reducing output impedance or reducing output ripple, but has an increased current consumption compared to a non-complementary type. Also, there is a problem in that, when the complementary drive is always performed, its conversion efficiency at the time of a low load or no load may be lowered.
Also, in the second conventional apparatus, it is designed taking in account of its maximum load for continuous operation, its output impedance and current consumption remain the same even when the load or input voltage state changes. Consequently, its capacity becomes excessive and is wasted at the time of a light load. In addition, there are problems in that the second conventional apparatus has a greater output ripple compared to the first conventional apparatus of the complementary driving type, and reduction of its impedance is difficult.
Furthermore, in power supply apparatuses for liquid crystal devices using DC/DC converters, it is desired to eliminate wastes in the current consumption, and improve the power conversion efficiency while maintaining an optimized display on a liquid crystal display device.
Accordingly, the present invention may provide a DC/DC converter that maintains a reduced output impedance, improves the efficiency in converting power at the time of a light load or no load, and realizes a higher power conversion efficiency.
The present invention may further provide a DC/DC converter that reduces current consumption at the time of a light load or no load to thereby eliminate wastes in the current consumption.
The present invention may still further provide a power supply apparatus for a liquid crystal device, which maintains an optimized display on a liquid crystal display apparatus, eliminate wastes in the current consumption and realizes a higher efficiency in power conversion.
A DC/DC converter according to one aspect of the present invention comprises:
two charge pump circuits each of which converts a DC input voltage into a given DC output voltage; and
two driving circuits driving the two charge pump circuits respectively,
wherein in a first mode, the two driving circuits complementarily drive the two charge pump circuits respectively, and the given output voltage is output from each of the two charge pump circuits, and
wherein in a second mode, one of the two driving circuits drives one of the two charge pump circuits, and the given output voltage is output from one of the two charge pump circuits and is not output from the other one of the two charge pump circuits.
In this aspect of the invention, two charge pump circuits are, for example, complementarily driven, and the driving of one of the charge pump circuits is controlled according to size of a load or the like. As a result, the low output impedance can be maintained and the efficiency in converting power at the time of a light load can be improved, whereby a higher power conversion efficiency can be realized.
A DC/DC converter according to another aspect of the present invention comprises:
a charge pump circuit which converts a DC input voltage into a given DC output voltage;
a driving circuit which drives the charge pump circuit;
an oscillation circuit which supplies an oscillation output to the driving circuit,
wherein the oscillation circuit varies a frequency of the oscillation output according to an operation mode.
In this aspect of the invention, the frequency of the drive signal for the charge pump circuit is varied according to size of a load or the like. As a result, the current consumption at the time of a low load can be reduced, and wastes in the current consumption can be eliminated.
A DC/DC converter according to a further aspect of the present invention comprises:
a charge pump circuit which converts a DC input voltage into a given DC output voltage;
a driving circuit which drives the charge pump circuit;
an oscillation circuit which supplies an oscillation output to the driving circuit,
wherein the charge pump circuit comprises:
a first switching circuit which includes a first transistor;
a second switching circuit which includes a second transistor having a smaller capability than the first transistor; and
a capacitor which is capable of changing a connecting condition by the first and second switching circuits,
wherein the driving circuit uses the first switching circuit to drive the charge pump circuit in a first mode, and uses the second switching circuit to drive the charge pump circuit in a second mode.
In this aspect of the invention, a transistor having a capability required according to size of a load or the like can be used and driven. As a result, the current consumption at the time of a low load can be reduced, and wastes in the current consumption can be eliminated.
A power supply apparatus for a liquid crystal device according to a still further aspect of the present invention comprises:
a first-stage charge pump circuit which converts a DC input voltage into a given DC output voltage;
a first-stage driving circuit which drives the first-stage charge pump circuit;
a series regulator which receives a DC output voltage of the first-stage charge pump circuit as an input voltage, and monitors an output voltage of the series regulator to output a constant voltage;
a second-stage charge pump circuit which steps up the output voltage of the series regulator by a given number of times;
a second-stage driving circuit which drives the second-stage charge pump circuit;
an oscillation circuit which oscillates at a given frequency;
a selection circuit which selects one of an oscillation output from the oscillation circuit and a display signal to be used for displaying on a display apparatus according to a selection signal; and
a timing signal generation circuit which generates a given timing signal to be supplied to each of the first-stage driving circuit and the second-stage driving circuit based on a signal that is selected by the selection circuit.
With the power supply apparatus according to this aspect of the invention, an oscillation output from the oscillation circuit or an external signal that has a lower frequency than the oscillation output and used for display on a display apparatus is selected according to size of a load or the like, and each of the charge pump circuits is driven based on the selected signal. As a result, the display of the display apparatus is optimized, wastes in the current consumption are eliminated, and a higher power conversion efficiency can be realized.